1. Field of the Invention
This invention relates to an objective lens for an optical disk, and more particularly to an objective lens composed of a single lens suitable for use with the case where the distance from a light source to an information recording surface is relatively small.
2. Description of the Prior Art
Recently, in the most general optical system used for a recording and reproducing device relative to an information recording medium such as an optical disk, light emitted from a light source 4 is formed into a parallel light by a collimator lens 3 and then condensed on an information recording surface 1 by an objective lens 2, as shown in FIG. 6. In the optical system of this kind, with respect to a surface deflection of an optical disk or the like, focussing is carried out by moving the objective lens 2 in a direction of an optical axis.
This system has a merit in that even if the objective lens is moved, the performance of the optical system remains unchanged, while the system gives rise to a problem in that two lenses, i.e., the objective lens 2 and the collimator lens 3, are required, and therefore the optical system becomes expensive.
On the other hand, in order to reduce the cost, vigorous studies have been progressed of a system in which light from a light source 4 is directly condensed on an information recording surface 1 by an objective lens 2 without using a collimator lens, as shown in FIGS. 7 and 8.
In the system shown in FIG. 7, focussing is carried out by movement of the objective lens 2 alone. The number of apertures and performance of the objective lens 2 are varied because of the movement of the objective lens 2, and therefore, the focussing magnification cannot be made so large. The reference focussing magnification was in the order of -1/40 to -1/8.
Recently, a compact disk reproducing optical system was reconsidered for the reasons below:
(1) Compactness of an optical system is requested.
(2) Because of an improvement in quality of a compact disk, even if the range within which focussing is possible is narrow, no practical problem occurs. As the result of this reconsideration, it becomes apparent that such an optical system can be used with the reference focussing magnification in the order of -1/4.
On the other hand, in the system shown in FIG. 8, focussing is carried out by moving a unit 5 of the entire optical system including a light source 4 and an objective lens 2. In this system, the variation of the numerical apertures and the deterioration of performance due to the focussing are not involved, but it is important to make small the distance from the light source 4 to the information recording surface 1 while securing the operating distance as required in order that the unit 5 may be reduced in weight as light as possible. For this reason, it is necessary to make the focussing magnification as large as possible, -1/6 to -1/2, as compared with the optical system shown in FIG. 7.
In reducing the cost in these optical systems, the system shown in FIG. 6 has a limit in that the objective lens 2 and the collimator lens 3 are respectively constituted by a single lens.
In the optical system shown in FIGS. 7 and 8, if the objective lens 2 is constituted by two lenses, the number of steps increases in incorporation and adjustment of lenses. So, the optical system shown in FIG. 6 is lower in cost than the others. Therefore, a single lens must be used.
Lenses developed for achieving the above-described object are known from Japanese Patent Laid-Open Publication Nos. 12661/1985, 56314/1986, 118708/1986, 177409/1986, 248014/1986, 252518/1986, 261711/1986 and 10613/1987.
Among those constituted as a single lens described above, Japanese Patent Laid-Open Publication Nos. 126616/1985 and 177409/1986 disclose a lens in which only the light source side is aspherical. It was found that such a lens is poor in the out-of-axis performance, and if in the optical system shown in FIG. 7, the objective lens is driven parallel to the disk to effect tracking, a reproducing signal becomes worsened. In case of the optical system shown in FIG. 8, the focussing magnification need be increased as described previously. However, in such a design, the out-of-axis characteristic becomes greatly worsened, and even when tracking is carried out by moving the unit 5, adjustment of the optical axis of the light source 4 and the objective lens 2 is difficult. Accordingly, in the lens in which one side is aspherical as described, the optical system capable of being used is considerably restricted.
On the other hand, the aspherical shape is expressed by a variety of methods. A method for expression by adding a compensating term represented by the even-number power expansion of the height from the optical axis to a term of a rotational secondary curved surface is most general. Among double aspherical lenses developed for achieving the above-described object, one disclosed in Japanese Patent Laid-Open Publication No. 56314/1986 is given a condition as to the amount of deformation from a reference spherical surface having the radius of curvature of the top at a position in the utmost periphery of an effective diameter of each surface, in which example, the aspherical surface is expressed in the form as described. However, the amount of deformation at the position of the utmost periphery of the effective diameter comprises a complicated total of conical coefficient and aspherical coefficient of each order. In the specific design, the respective coefficients have a large freedom, and even if this condition is fulfilled, there often results in a lens which is large in an aspherical term of high order involving a difficulty in processing and which is large in sensitivity with respect to the parallel eccentricity between the surface on the light source side and the surface opposite to the light source.
The lenses disclosed in Japanese Patent Laid-Open Publication Nos. 118708/1986, 248014/1986, 252518/1986, 261711/1986 and 10613/1987 define the conical coefficient of the surface on the light source side. The compensating term from the secondary curved surface uses the term proportional to the tenth power of the height from the optical axis. However, particularly if the magnification increases, the aspherical amount also increases, and therefore, it is desireable in processing to have an aspherical shape without use of a high-order aspherical term.